Metal forming



Dec. 29, 1964 R. H. WESLEY ETAL METAL FORMING Original Filed May 25, 1959 INVENTORS [ZlCHAQD H. WESLEY DONALD 6 BY COLE.

Me /mm ATTORNEY United States Patent 3,163,141 METAL FQRMING Richard H. Wesley, Fort Worth, and Donald W. Cole,

Weatherford, Tern, assignors to General Dynamics Corporation, San Diego, Calif., a corporation or Delaware Continuation of application Slern No. 815,380, lday 25, 1959. This application July 15, 1963, Ser. No. 294,995 1 Claim. (Cl. 113-44) This application is a continuation of application Serial No. 815,380, filed May 25, 1959, now abandoned.

The present invention relates generally to the formation of materials; and more particularly, to a method of forming materials into desired shapes wherein the energy for such formation is produced by a high energy electrical spark.

The design of present day aircraft often requires the use of components whose configurations are inherently diflicult to form by conventional methods. Moreover, such parts are often of small dimensions and are fabricated of materials which strongly resist such formation. As a result of such requirements it has become necessary to develop new methods which are particularly adapted to this type of formation. The provision of such a method is the objective of the present invention.

Heretofore, the formation of such hard-to-form parts has generally been accomplished by utilizing the pressures derived from the explosion of high energy, chemical explosives such as nitroglycerin and trinitrotoluene.

Several disadvantages are inherent in such a method. As a result of the chemical nature of explosive materials, it is extremely diflicult to control the energy output of a given charge. The location of the site at which such operations are carried out must be remote from populated areas, dwellings and building which might be damaged by the explosions. Another objection is that special facilities must be provided for the storage of the explosives and detonator caps. The preparation for a forming operation of this type is often time consuming and must be done by personnel especially trained in the handling of explosives.

The present invention resides in a method of forming ductile materials into desired shapes wherein a workpiece is caused to assume the precise configuration of a die as a result of the pressure applied thereupon by means of a shock wave generated by a high energy electrical spark discharge within a pressure transmitting medium.

This method offers several advantages over the explosive forming method. The energy output utilized in forming may be controlled precisely. This method may be employed with safety at any location where there is an electrical power source available. The preparation for an operation of this type is simple, rapid and may be done by personnel having a minimum amount of training, and because of the low cost of electrical power, this method may be practiced very inexpensively.

It is, therefore, an object of the invention to provide a method of forming ductile materials into desired shapes, which method utilizes the energy derived from an electrical spark discharge in a pressure transmitting medium.

Another object is to provide a method of forming ductile materials into desired shapes which afiords precise control of the energy utilized in forming.

Still another object resides in the provision of a method of forming ductile materials into desired shapes, which method may be practiced by personnel having a minimum amount of technical training.

A further object of the invention is the provision of a method of forming materials into desired shapes which is simple, rapid and economical to practice.

These and other objects and advantages of the invention will be more apparent to those skilled in the art from the following description of the appended drawings wherein:

Bdhfildl Patented Dec. 2%., 1964 FlGURE 1 is a cross-sectional view taken through a typical apparatus employed in the present invention; and

FIGURE 2 is a block diagram showing the electrical apparatus for producing the high energy spark.

Referring now to the drawings and in particular to FIGURE 1, the apparatus associated with the invention, generally indicated by the numeral Ill), basically comprises a workpiece 11 to be formed, a die 12 of desired configuration, a draw ring 13, a body of pressure transmitting medium 14, and a pair of electrodes 15 having a spark gap 16 therebetween.

The workpiece 11 is first assembled with the die 12 by positioning it over die cavity 18. Prior to such assembly, the die .cavity is generally coated with a thin film of a light lubrication substance to facilitate removal of the workpiece after forming. The workpiece is of the same general shape as the opening of the die cavity but of larger areal dimension, so that it extends beyond the upper perimeter of the die cavity, as indicated at 19. In order to secure the workpiece in position the draw ring 13 is placed thereupon and fastened to the die by means of a plurality of threaded studs 21 which extend through the draw ring and engage with a plurality of threaded holes 22 in the die 12. A step 23 is provided in the draw ring to compensate for the thickness of the workpiece. When the studs 21 are adequately tightened, a seal is effected between the workpiece and the upper surface of the die and a restraining pressure is applied to the workpiece, the purposes for which will become clear below.

When the workpiece has been assembled with the die, as decribed above, the assembly is submerged in the body of pressure transmitting medium 14-. Such pressure transmitting medium is preferably a liquid, such as water, but in some cases, where the workpiece consist of a relatively thin sheet of highly ductile material, a gas, such as ordinary air, will sufiice as such a medium.

The electrodes 15 are then positioned in the pressure transmitting medium a predetermined distance above the approximate center of the workpiece. Such distance is determined by several factors, including: the ductility of the workpiece material, the thickness of the workpiece, the depth and degree of formation required, the density of the pressure transmitting medium and the power output at the spark gap. In general it has been found that the distance of the electrodes from the workpiece should be directly proportional to the ductility of the workpiece material, the density of the pressure transmitting medium and the power output, and inversely proportional to the workpiece thickness and the depth and degree of formation.

The high energy spark required at spark gap 16 is furnished by the circuit illustrated in FIGURE 2. Power from a suitable A.C. source is applied to the primary winding of a voltage step-up transformer 25. Input voltage may be controlled by a variable auto-transformer 26 in the primary circuit or" transformer 26. The high voltage secondary winding of transformer 25 is connected to a full wave bridge rectifier 27. Direct voltage from bridge rectifier 27 is applied across a condenser bank 31 through a charge switch 32. Condenser bank 31 includes a plurality of condensers 33, $4 and 35. Each condenser may be selectively connected in circuit by one of switches 36, 3'7 and 4-1. Condenser bank 31 is connected to electrodes 15 through a discharge switch 42.

In order to charge the condenser bank, discharge switch 42 is opened, charging switch 32 and selected ones of condenser switches 36, 37 and 41 are closed. Autotransformer 26 is adjusted to provide the desired DC. voltage to charge the selected condenser or condensers. After the desired charge has been placed in the condenser bank, charge switch 32 is opened and discharge switch 42 is which, upon striking the workpiece, forces it into the die cavity 18. The outer perimeter of the workpiece is held in position by the restraining pressure exerted thereon by the draw ring 13. Such pressure is increased during the forming of the workpiece as a result of the shock wave striking the draw ring and workpiece at substantially the same instant. It is very important that the seal between the workpiece and the die be adequate to prevent the leakage of the pressure transmitting medium into the die cavity, since the presence thereof would impede the satisfactory formation of the workpiece.

As an example of the present method, in the formation of a circular titanium workpiece, eight inches in diameter and of .032 thickness, to be formed to the configuration of the die cavity 18 of maximum depth of two inches, it has been found by experiment that satisfactory formation occurs with a power output of one and one-half million watts in water as a medium at a distance of Six inches above the center of the workpiece. It has also been found that in general by heating the workpiece, the formation thereof requires less energy. Such heating may be effected by circulating a hot oil or molten salt through a series of coils 30 in the die 22. For example, in the present instance by heating the titanium workpiece to apprommately 300 F., satisfactory formation is achieved with the same distance between the electrodes and workpiece with an energy output of: only one million :atts; or, as an alternative, the distance may be increased to eight inches, holding the power output at one and onehalf million watts.

The only requirement for a satisfactory pressure transmitting medium is that it transmit the shock wave and, therefore, the shock wave pressure' The shock wave velocity and forming efficiency is increased by increasing the density of the pressure transmitting medium, with the result that a smaller power output is required to do a particular forming job. However, this is not desirable in all cases, since the increased shock wave velocity imparts a proportionately higher pressure pulse to the workpiece, resulting in a greater lateral accelration along the horizontal surface which very often causes an overformation at the bend radii.

in most cases the forming operation can be accomplished with a single discharge of the condenser bank. However, in forming operations involving inductile alloys, difiicult shapes or heavy gauges, it is sometimes necessary to charge and discharge the condenser bank several times in order to obtain the desired formation.

in some cases it is desirable to evacuate the air entrapped in the die cavity prior to forming order to obtain the proper formation. Such evacuation may be effected through an aperture (not shown) into the die cavity to which a vacuum pump is attached. Such aperture generally contains a porous metal plug which is flush with the surrounding die cavity surface to prevent the workpiece from being formed thereinto.

From the above description of the present forming method it is apparent that the invention is also readily adaptable to such associated operations as blanking, embossing, etc. These latter operations are accomplished by merely substituting an appropriate die for the above described die 12.

As thus described the present invention is characterized as a simple, rapid, economical and novel method of forming materials into desired shapes wherein a workpiece is caused to assume the shape of a die by means of a lu'gh energ' electrical spark produced in a pressure transmitting medium wherein the workpiece and die are submerged.

Although only the preferred embodiment of the invention has been herein described, it is to be understood that the invention is not limited thereto, as many variations and modifications will be apparent to those skilled in the art, and the invention is to be given its broadest possible interpretation Within the scope and terms of the following claims:

What we claim is:

A metal forming device comprising an open-topped liquid container, a liquid contained within said container, a concave die immersed in said liquid in said container, means for securng a sheet metal workpiece to said die, said means including a draw ring shaped to conform to the upper edge of said die and the periphery of said sheet metal workpiece, a plurality of studs in peripherally distributed apertures in said draw ring cooperating with tapped apertures in the upper edge of die, hot fluid passages in said die to heat said die and said workpiece, high energy spark producing means including a source of alternating current, a variable voltage autotransr'orrner connected to said source of alternating current, a voltage step-up transformer having a low voltage primary winding connected to said autotransformer and a high voltage secondary winding, a full-wave rectifier connected to said high-voltage secondary winding, a plurality of capacitors, a switch connected to each of said capacitors for connecting selected capacitors in parallel, a charging switch connected to said full-wave rectifier for connecting said selected capacitors to said rectifier, a pair of electrodes defining a spark gap immersed in said liquid adjacent said workpiece, a discharger switch connected to said selected capacitors and to said electrodes to discharge said selected capacitors through said sparr gap and generate a shock wave in said liquid.

References fitted by the Examiner UNITEB STATES PATENTS 781,619 1/05 Rogers ll344 2,271,890 2/42 Rehhan 317-44 2,299,941 10/42 Townsend 3 l7-l5l 2,348,921 5/44 Pavlecka l l35'1 2,395,600 2/46 Weisglass 3l7-l51 2,518,518 8/50 Beldi 3l7-l4 2,594,520 4/52 Tiedrnari 3l715l 2,935,038 5/60 Chatten 113-44 2,969,758 1/61 Hewlett et al. lift-44 FOREIGN PATENTS 742,460 12/ 5 5 Great Britain.

105,422 9/42 Sweden.

252,625 10/48 Switzerland.

OTHER REFERENCES Martin, E. A.: The Underwater Spark; An Example of Gaseous Conduction of About 10,000 Atmospheres, July 1956.

Dissertation Abstracts: pages ll0l l 1, vol. 19, July 1953.

CHARLES W LANHAM, Primary Examiner. 

